I have searched thoroughly, a lot, so i can answer VALIDLY. Here goes...

All DLCs sold here, is the complete package. Season pass on steam is broken down into 3-4 parts here, listed separately and GOG version contains an armor set, exclusive to steam's season pass (maker armor), so even if it is not listed, it is actually present and activated. Also, in game, in screen with options/continue/DLC etc, if you press the DLC tab, the one you choose the 3 extra DLC campaigns from (argul's tomb etc), it says: "ALL DLC UNLOCKED".

1) Crow Armor had been a promo event. You had to log into THQ server from the Book of the Dead in-game mailing system and by doing so, you got this armor set. There is no other way and event is well over by now. Hell, even THQ has disbanded and its server is no more, either. This was not a DLC. Promo event and methinks, with time limit period, too. This is not redeemable anymore, not even at steam.

2) Pauldron of the Horseman is an item given to you in Darksiders 2, if you PLAYED Darksiders 1 before 2. In order for it to unlock in-game (shaman npc in tri-stone would give it), you have to get 1 random achievement from Darksiders 1 (for the game-client to verify you played the first game). Galaxy has no achievements (yet), this works, but ONLY in steam. This item serves mostly a cosmetic role, though. Other DLC armor/weapons-set present here, offer much better items, stats-wise, appearance-wise, etc. Still, i really wanted to wear that.

3) Chaos Fang Scythe is an item given to you in Darksiders 2, if you FINISHED Darksiders 1 before 2. In order for it to unlock in-game (shaman npc in tri-stone would give it), you have to get the achievement for beating the game from Darksiders 1 (for the game-client to veryfy you beat the first game). Galaxy has no achievements (yet), this works, but ONLY in steam. This item serves mostly a cosmetic role, though. Other DLC armor/weapons-set present here, offer much better items, stats-wise, appearance-wise, etc. Still, i really wanted to wear that.

Those are not DLCs, however, so the statement of GOG that this DLC collection is complete, is the truth, really. It even contains the Crucible (arena like gameplay with waves sent against you). Hopefully, in the future, when achievements get implemented in Galaxy, at least the ones for having played and beaten Darksiders 1, item rewards, might appear for us in-game. Still, though, gog version is by far a sweeter deal than steam. No DRM, no obligatory client, no connectivity required, nothing at all. I suggest you get them here.

IMPORTANT! GOG version actually contains 2 unique, exclusive items, that back in the day, they both were exclusive bonus for PREORDERING the game! Advantage over steam version! Dark Talons (Claws) - Skeletal Axe of Rending (Axe)